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Old 5th Nov 2023, 4:24 pm   #1
pauric hennessy
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Default Defiant MSH453 repair

Hi there, my wife's had the above radio for 20 years in her clinic. About 5 years ago it sopped working -no audio . I kept it for last 5 years with good intentions to try to repair.Only opened it yesterday. So far I can'tin a line are illuminating, There is a larger 6P25 valve that is not illuminating,but its a different type , almost like an early day low energy bulb. Does anyone have access to schematics for this model or can anyone offer advice on how to proceed with repair. I've good knowledge of electronics. I have a multimeter and soldering iron for any component replacement. Whats best way of debugging these circuits?
I can post some pictures if it would help
thanks in advance

Pauric
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Old 5th Nov 2023, 5:06 pm   #2
paulsherwin
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Default Re: Defiant MSH453 repair

The 6P25 is a large audio output valve widely used in the late 40s and 50s. You should be able to see a heater glow, though it may not be very bright or obvious as the valve is coated. If there really is no glow, remove the valve and measure the resistance between the heater pins (2 & 7). If you find a large or infinite resistance, then the heater has failed open circuit. Note that this is an unusual failure mode in normal use.

http://www.r-type.org/exhib/aas0037.htm

Defiant was the Co-op house brand, and the chassis was probably made by Plessey who did a lot of OEM work.
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Old 5th Nov 2023, 6:11 pm   #3
Silicon
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Default Re: Defiant MSH453 repair

Has anyone replaced the waxed paper capacitors in this radio?

There are usually several that need to be replaced Before using a vintage radio like this.

A failure of the 'interstage coupling capacitor' can damage output valves, output transformers and even mains transformers.
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Old 5th Nov 2023, 6:22 pm   #4
paulsherwin
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Default Re: Defiant MSH453 repair

Yes, it's likely to need further work even after the immediate fault is identified and fixed. You will need to decide if you want to do this yourself, pay somebody else, or keep the radio in a non functional state. There isn't much to listen to on MW and LW nowadays - I think all the RTÉ transmitters have closed, and the remaining UK ones are on borrowed time.
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Old 5th Nov 2023, 6:32 pm   #5
bigfathairyvika
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Default Re: Defiant MSH453 repair

The 6P25 valve in my defiant has a clear top through which the top of the heater is easily seen.

I used the Murphy A170 schematic as it is the nearest I could find.

Only differences are in the arrangement of the antenna coupling and tuning circuits.
All the components values should be taken from the markings on the actual ones fitted as values differ slightly from the schematic.
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Old 5th Nov 2023, 7:30 pm   #6
Silicon
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Default Re: Defiant MSH453 repair

There is a lot of useful information on the 'vintage-radio.com' website.

A 'Lamp limiter' is a useful device for initially applying power.

I would start by measuring the resistance across the Live and Neutral pins of the mains plug.

If the on-off switch is faulty you won,t get any power. You should get a resistance in the region of 50 Ohms.

If the primary circuit seems OK, remove the rectifier valve and apply power and see if the valve heaters glow.
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Old 20th Nov 2023, 6:12 pm   #7
pauric hennessy
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Unhappy Re: Defiant MSH453 repair

Quote:
Originally Posted by paulsherwin View Post
Yes, it's likely to need further work even after the immediate fault is identified and fixed. You will need to decide if you want to do this yourself, pay somebody else, or keep the radio in a non functional state. There isn't much to listen to on MW and LW nowadays - I think all the RTÉ transmitters have closed, and the remaining UK ones are on borrowed time.
Thanks Paul, was out of country since 10th Nov but back now. I take on board your comment about all RTE transmitters have closed down on MW and I think LW252 closed down in April 2023. So there is little point in repairing the radio if all that can be heard is static white noise!!
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Old 20th Nov 2023, 6:18 pm   #8
pauric hennessy
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Default Re: Defiant MSH453 repair

Quote:
Originally Posted by paulsherwin View Post
The 6P25 is a large audio output valve widely used in the late 40s and 50s. You should be able to see a heater glow, though it may not be very bright or obvious as the valve is coated. If there really is no glow, remove the valve and measure the resistance between the heater pins (2 & 7). If you find a large or infinite resistance, then the heater has failed open circuit. Note that this is an unusual failure mode in normal use.

http://www.r-type.org/exhib/aas0037.htm

Defiant was the Co-op house brand, and the chassis was probably made by Plessey who did a lot of OEM work.
Hi Paul , I was mistaken about the 6P25, it is glowing. Does a glowing valve generally indicate that its probably ok, the gate , cathode and anode of these
valves could have problems I guess as well
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Old 20th Nov 2023, 10:35 pm   #9
Simon Gittins
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Default Re: Defiant MSH453 repair

The glowing valve is a good sign, it can't work if it isn't glowing. However, as you have guessed, a glowing valve can have other problems although they are more reliable than most people imagine.
A good place to start reading is:
https://www.vintage-radio.com/repair...ion/index.html
and in particular:
https://www.vintage-radio.com/repair...lves-work.html
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Old 21st Nov 2023, 1:54 am   #10
Silicon
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Default Re: Defiant MSH453 repair

If you download the schematic of the Murphy A170 you will see a capacitor linking the anode of the 6LD20 valve to the grid of the 6P25 output valve.

This is usually a waxed paper capacitor. It is 70 years old and will leak a small amount of current and upset the bias on the output valve. This could then cause other damage.

Please replace it before powering up the radio.

It is usually 0.01uF and rated for 400V.

The Murphy service sheet will show the connections on the valve holder underneath the chassis.
Other websites such as 'R-type.org' will give you the pin allocation for the grid, cathode and anode, etc.

Measure the resistance of the primary of the output transformer (with receiver disconnected from mains) and tell us what you get. It may be easier for you to measure the resistance from pin 3 to pin 4 on the valve holder of the 6P25.
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Old 26th Nov 2023, 4:08 pm   #11
pauric hennessy
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Default Re: Defiant MSH453 repair

Thanks Silicon, need to get a meter to follow your debug strategy .I will let you know how I get on. Considering what Paul said about the diminishing MW/LW transmitters in UK and none I think in Ireland, even if I get it repaired it's of little use as a radio in Ireland- would you agree? Is there any way of upgrading it to try and receive FM? Any advice would be greatly appreciated. I know that FM is from 80MHz to 107MHz, and operated on frequency modulation where as MW is operating around the 1MHz band

Regards, Pauric.
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